The present invention relates to an image sensing apparatus for sensing an image of an object.
In a conventional image sensing apparatus which uses solid image sensing elements, such as CCD, arranged in two dimension, to output an image to be used by a printer or applied with image processing by a computer, the total number of pixel is less than that of an image sensing apparatus, such as a scanner, using image sensing elements arranged in one dimension. Accordingly, there is a problem that resolution of an obtained image is low, thus an output image of high quality can not be obtained. In order to overcome the above problem, a method in which an imaging position of an optical image on a photo-sensing surface of a solid image sensing element is shifted by half pixel in the vertical and horizontal directions so as to obtain four frames of the image, then they are read and processed into a single image thus obtaining an image of high resolution expressed by four times as many pixels as the image expressed by a single frame, has been developed.
For example, as for improving an apparent resolution of an image, the Japanese Patent Laid-Open No. 64-69160 discloses a method of shifting CCD in the horizontal and vertical directions by using a piezoelectric element, and the Japanese Patent Laid-Open No. 63-284979 discloses a method of changing the light path by using-an optical device provided between an image sensing lens system and CCD. Further, in the Japanese Patent Laid-Open No. 4-311448, a variable angle prism (VAP) is used as means for changing the light path.
According to the aforesaid signal processing method of the aforesaid image sensing apparatus, the apparent resolution improves, however, the following problem remains. Namely, the maximum dynamic range of luminance change in the nature is about 100,000 luxes, whereas there is a limitation in the range of luminance change that a solid image sensing element, such as CCD, can reproduce.
Accordingly, in a conventional video camera or the like, the intensity of light which illuminates a solid image sensing element is controlled by using an iris diaphragm provided in front of the solid image sensing element. Further, in a case where the intensity of incoming light is not enough and the output signal level does not reach a predetermined value even when the iris diaphragm is completely opened, the signal level is amplified by an automatic gain control circuit.
However, in the aforesaid conventional image sensing apparatus, in a case where the output from an image sensing element is amplified by a large gain when the object is not illuminated enough, noises are also amplified accordingly, which deteriorates S/N ratio of an obtained image signal. Therefore, the processed image could not have high quality.
There is a conventional method often used against the aforesaid problem, which makes reading speed of a solid image sensing element slower so that the solid image sensing element can be charged enough even when luminance of an object is not satisfactory. However, noises occurred in a solid image sensing element of CCD or the like increase as an amount of charge increases, thus the S/N ratio does not improve as desired.
Furthermore, if the intensity of light which illuminates an object changes while sensing four frames of a still image, values of corresponding pixels in the four frames differ when the four frames are processed. The difference becomes a carrier component which deteriorates image quality.